# Water Hammer **URL:** \ **Category:** Factual Questions\ **Created:** [March 30, 2011, 6:40pm UTC](https://boards.straightdope.com/t/water-hammer/576539 "2011-03-30T18:40:27Z")\ **Posts on this page:** 18\ **Page:** 1
**Author:** ![Lare](https://avatars.discourse-cdn.com/v4/letter/l/53a042/32.png) [@Lare](https://boards.straightdope.com/u/Lare)\ **Post date:** [March 30, 2011, 6:40pm UTC](https://boards.straightdope.com/t/water-hammer/576539/1 "2011-03-30T18:40:27Z")
(Now be honest: Doesn’t “water hammer” sound like it would be a heck of a cool tool? I picture driving in nails with a souped-up squirt gun, but I digress…) The (foreclosed-and-abandoned) house I bought has an issue with water hammer. Whenever a toilet re-fills, or someone shuts off a faucet, the resultant “bang!” is annoying at least, and a precursor to disaster at most. I started by adding a regulator to the city line, bringing the incoming pressure from 90 to about 45. This helped some. Next I tried “water hammer arresters” (typical one [shown here](http://www.google.com/products/catalog?q=water+hammer+arresters&oe=utf-8&rls=org.mozilla:en-US:official&client=firefox-a&um=1&ie=UTF-8&cid=4373672689626390701&sa=X&ei=THeTTf2vMIqCsQOKu5i-BQ&ved=0CCMQ8gIwAA#)). I installed one at each toilet and one on the supply line for the tub. Again, helped some more, but still not enough. When any of the arrester-equipped devices are shut off, there’s still a bang. The pipes aren’t moving, they’re fastened to studs with screwed-down clamps (not the hammered-in type). I’m not sure what to try next. For the sake of the money, I’m trying not to involve a plumber. Any ideas that don’t involve re-piping? ---
**Author:** ![Leaffan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leaffan/32/299_2.png) [@Leaffan](https://boards.straightdope.com/u/Leaffan)\ **Post date:** [March 30, 2011, 6:50pm UTC](https://boards.straightdope.com/t/water-hammer/576539/2 "2011-03-30T18:50:29Z")
Isn’t a blocked [vent](http://en.wikipedia.org/wiki/Drain-waste-vent_system) sometimes a cause of water hammer? ---
**Author:** ![Lare](https://avatars.discourse-cdn.com/v4/letter/l/53a042/32.png) [@Lare](https://boards.straightdope.com/u/Lare)\ **Post date:** [March 30, 2011, 6:57pm UTC](https://boards.straightdope.com/t/water-hammer/576539/3 "2011-03-30T18:57:41Z")
Maybe? But at least in my case, (and for an unrelated reason) I had the stack checked and they were clear on both sides of the house. My hammer occurs when the valve is shut off. I’m seriously considering trying to find the older style fill valves that did so slowly instead of snapping shut but I know this is just maksing the problem instead of fixing it. ---
**Author:** ![Sparky812](https://avatars.discourse-cdn.com/v4/letter/s/eb9ed0/32.png) [@Sparky812](https://boards.straightdope.com/u/Sparky812)\ **Post date:** [March 30, 2011, 7:10pm UTC](https://boards.straightdope.com/t/water-hammer/576539/4 "2011-03-30T19:10:36Z")
> [@Lare](#): > > The pipes aren’t moving, they’re fastened to studs with screwed-down clamps (not the hammered-in type). The pipes may not be visibly moving but I would guess that they are moving and/or flexing enough to cause the bang you hear. Can you narrow down where the bang is coming from? I would check your pipes to your shower controls and head which are common sources of water hammer noise. Also, you may try replacing the whole toilet tank assembly, specifically the refill valve. > [@Leaffan](#): > > Isn’t a blocked [vent](http://en.wikipedia.org/wiki/Drain-waste-vent_system) sometimes a cause of water hammer? No, it does not. ---
**Author:** ![Leaffan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leaffan/32/299_2.png) [@Leaffan](https://boards.straightdope.com/u/Leaffan)\ **Post date:** [March 30, 2011, 7:16pm UTC](https://boards.straightdope.com/t/water-hammer/576539/5 "2011-03-30T19:16:45Z")
> [@Sparky812](#): > > …No, it does not. Well, the first [link](http://en.wikipedia.org/wiki/Water_hammer) I just clicked on disagrees: :dubious: > [@](#): > > To prevent this, air and vacuum relief valves, or air vents, are installed just downstream of the valve to allow air to enter the line and prevent this vacuum from occurring. Of course it is only Wikipedia. ---
**Author:** ![Lare](https://avatars.discourse-cdn.com/v4/letter/l/53a042/32.png) [@Lare](https://boards.straightdope.com/u/Lare)\ **Post date:** [March 30, 2011, 7:59pm UTC](https://boards.straightdope.com/t/water-hammer/576539/6 "2011-03-30T19:59:13Z")
> [@Sparky812](#): > > The pipes may not be visibly moving but I would guess that they are moving and/or flexing enough to cause the bang you hear. > > Can you narrow down where the bang is coming from? > I would check your pipes to your shower controls and head which are common sources of water hammer noise. > Also, you may try replacing the whole toilet tank assembly, specifically the refill valve. > > No, it does not. As part of the work I had to do, I fastened down the pipes myself. The one I was most suspicious of can’t be moving. (Not to say another one isn’t.) The house was built in '85. At the shower they had done the old “copper T to a capped off pipe about one foot long” as an arrester. Besides trying to drain the whole system down to replace the air in those pipes (didn’t help) that’s exactly where I put the arresters, cutting out the air pipe. The toilets are brand new, I had to put them in before I could move in. 🙂 Do you still think I should suspect them, though? ---
**Author:** ![Sparky812](https://avatars.discourse-cdn.com/v4/letter/s/eb9ed0/32.png) [@Sparky812](https://boards.straightdope.com/u/Sparky812)\ **Post date:** [March 31, 2011, 11:30am UTC](https://boards.straightdope.com/t/water-hammer/576539/7 "2011-03-31T11:30:14Z")
> [@Leaffan](#): > > Well, the first [link](http://en.wikipedia.org/wiki/Water_hammer) I just clicked on disagrees: :dubious: > > Of course it is only Wikipedia. It does not disagree, the diagram in your first link depicts a drainage system. A blocked vent on your main stack will not cause water hammer when you turn your taps on or off or flush the toilet. Secondly, the vent in your Wikipedia link is more like [this](http://www.maid-o-mist.com/autoair.html). ---
**Author:** ![raindog](https://avatars.discourse-cdn.com/v4/letter/r/2bfe46/32.png) [@raindog](https://boards.straightdope.com/u/raindog)\ **Post date:** [March 31, 2011, 12:00pm UTC](https://boards.straightdope.com/t/water-hammer/576539/8 "2011-03-31T12:00:41Z")
Do you have an expansion tank on the water heater? ---
**Author:** ![Lare](https://avatars.discourse-cdn.com/v4/letter/l/53a042/32.png) [@Lare](https://boards.straightdope.com/u/Lare)\ **Post date:** [March 31, 2011, 1:57pm UTC](https://boards.straightdope.com/t/water-hammer/576539/9 "2011-03-31T13:57:56Z")
> [@raindog](#): > > Do you have an expansion tank on the water heater? No–but I also have not appreciable hammer on the hot water taps–it’s the cold ones that make me jump. ---
**Author:** ![Lare](https://avatars.discourse-cdn.com/v4/letter/l/53a042/32.png) [@Lare](https://boards.straightdope.com/u/Lare)\ **Post date:** [March 31, 2011, 2:07pm UTC](https://boards.straightdope.com/t/water-hammer/576539/10 "2011-03-31T14:07:08Z")
> [@Lare](#): > > No–but I also have not appreciable hammer on the hot water taps–it’s the cold ones that make me jump. Whoa. Too early in the morning… It should have said: No–but I also don’t have appreciable hammer on the hot water taps–it’s the cold ones that make me jump. ---
**Author:** ![Sparky812](https://avatars.discourse-cdn.com/v4/letter/s/eb9ed0/32.png) [@Sparky812](https://boards.straightdope.com/u/Sparky812)\ **Post date:** [March 31, 2011, 2:27pm UTC](https://boards.straightdope.com/t/water-hammer/576539/11 "2011-03-31T14:27:15Z")
Ok, that’s progress you’ve narrowed it down to just the cold water lines. Is it just one branch? i.e. the shower and toilet are on the same supply line How many 90 degree turns or Ts are on that line? Are they anchored securely in all 3 dimensions? Water hammer will move loose sections of pipe along their length and transfer the energy to any fittings, structure or fixtures. Are there any shutoffs in that line? Check your washers are any valves or taps. Have you checked the compensating valve at your shower controls? ---
**Author:** ![raindog](https://avatars.discourse-cdn.com/v4/letter/r/2bfe46/32.png) [@raindog](https://boards.straightdope.com/u/raindog)\ **Post date:** [March 31, 2011, 5:19pm UTC](https://boards.straightdope.com/t/water-hammer/576539/12 "2011-03-31T17:19:42Z")
The expansion tank is on the cold water side. While they are principally for expansion, they serve another function for water hammer. They are common on systems that have high incoming water pressure. I’d be installing an expansion tank. ---
**Author:** ![Lare](https://avatars.discourse-cdn.com/v4/letter/l/53a042/32.png) [@Lare](https://boards.straightdope.com/u/Lare)\ **Post date:** [April 4, 2011, 6:04pm UTC](https://boards.straightdope.com/t/water-hammer/576539/13 "2011-04-04T18:04:13Z")
Maybe I’ll try to find one, too. Thanks. ---
**Author:** ![Harland1](https://avatars.discourse-cdn.com/v4/letter/h/7feea3/32.png) [@Harland1](https://boards.straightdope.com/u/Harland1)\ **Post date:** [July 13, 2011, 6:17pm UTC](https://boards.straightdope.com/t/water-hammer/576539/14 "2011-07-13T18:17:19Z")
Guys,. Water hammering is caused by entrained air in your plumbing system. With no place to go it just builds pressure until by force it escapes,but not without beating the crap out of your pipes on it’s way out. Install an expansion chamber in the wall or very near to your toilet., ---
**Author:** ![Lare](https://avatars.discourse-cdn.com/v4/letter/l/53a042/32.png) [@Lare](https://boards.straightdope.com/u/Lare)\ **Post date:** [July 13, 2011, 6:22pm UTC](https://boards.straightdope.com/t/water-hammer/576539/15 "2011-07-13T18:22:23Z")
n/m ---
**Author:** ![crawfyssj5](https://avatars.discourse-cdn.com/v4/letter/c/46a35a/32.png) [@crawfyssj5](https://boards.straightdope.com/u/crawfyssj5)\ **Post date:** [July 14, 2011, 1:30pm UTC](https://boards.straightdope.com/t/water-hammer/576539/16 "2011-07-14T13:30:31Z")
Just had an excellent idea for a water hammer, if someone made a material strong enough and fill it with water, then this might be quieter and also give more force in a Downward hit. This would probably be good for a sledge hammer 😃 ---
**Author:** ![ankh](https://avatars.discourse-cdn.com/v4/letter/a/ea5d25/32.png) [@ankh](https://boards.straightdope.com/u/ankh)\ **Post date:** [July 15, 2011, 5:00pm UTC](https://boards.straightdope.com/t/water-hammer/576539/17 "2011-07-15T17:00:18Z")
Here’s what our plumber taught us when we bought a 70 year old house: The water hammer happens because you have a large weight of water moving through the water pipes (figure the volume – length x diameter). Water isn’t compressible. When the faucet/washer valve closes suddenly, all the momentum of that water has to go somewhere, so some part of the pipe system moves sideways and bangs against the inside of a wall. There are almost certainly some ‘arresters’ built into the walls – little vertical stub pipes that dead-end, put in by the original plumber. Those, at first, were filled with air and that acts as a shock absorber (air compresses; water doesn’t). But air is soluble in water, and more soluble in cold water as it comes into the system. Over the years, the air in those stub pipes slowly dissolves into the water, meaning the water fills the stub pipes. Solution is – turn off the water at the main inlet or the meter. Then attach a drain hose to the lowest faucet in the system so the water you’re draining out goes somewhere convenient. Open the cold water taps throughout, and let the water all drain out of the pipes; wait a while til it stops running out. Then close that lowest valve and turn the main water valve back on. You’ll get a whole lot of noise and gurgling and splashing at each faucet. You’ll also need to flush the toilets a few times so you don’t scare the next user – same thing, they’ll be noisy til the air’s pushed out of all the pipes. If the plumbing was built in the usual way, you won’t hear the water hammers again for a few years. (If that doesn’t work, it will at least let you isolate the problem – have someone turning the faucets while you hang out in the basement til you know which pipe is still making the water hammer bang, and add one of those arrester things to that part of the system.) This is also a good time to flush the sediment out of the bottom of your water heater. ---
**Author:** ![ankh](https://avatars.discourse-cdn.com/v4/letter/a/ea5d25/32.png) [@ankh](https://boards.straightdope.com/u/ankh)\ **Post date:** [July 15, 2011, 5:05pm UTC](https://boards.straightdope.com/t/water-hammer/576539/18 "2011-07-15T17:05:14Z")
p.s. – as you noticed you don’t hear the water hammer in the hot water pipes. Notice what happens when you put a glass of cold water on the table — as it warms up bubbles form. That’s the dissolved air coming out of solution. So odds are that on the hot water branches of the system there have been enough bubbles to keep the little stub pipes properly holding their bubble of air. It’s the cold water side where the air in the stub pipes slowly disappears into solution.